Does anyone know if one with an L permit is allowed to rent second apartment in a different canton (ZH, currently residing in Zug), in order to, say, stay there 2-3 nights a week, and whether that will/will not cause further tax/permit complications?

I've no idea on the implications for an L permit holder but would it not be cheaper to rent a room in a shared apartment?

It all depends on your L permit. If it is an EU L AND states on the back that it is valid for the whole of Switzerland then yes you can. The tax is defined as payable in the place that is the centre of your life ie your main residence. Note that this need not be the place where you reside most of the time.

mine is not EU and it is only valid for Zug - it's stuck with the employer and that's where I work and supposedly have "the centre of my life"
so I am assuming there should be no problem as long as apartment's owner is ok with me having this "extra-temporary" permit and will actially lease it to me?

a room in a shared aparment is not for me - as Richard said, I might end up spending most of my time in Zurich

Did you check the back of your permit like Richard said? I think you'll see that it tells you that you are not allowed to move from the canton where you work without permission. That should answer your question fairly quickly.

Remember that many people try to do this to save taxes - therefore the canton that issued your permit is keen not to lose tax revenue.

well, mark, that DOES NOT answer my question, as if you read it carefully you'll see that I'm not going to move, and am going to keep my current apatment and part of my activities in Zug (work would be a good example ). all I'm plannign is to use another apartment in Zurich as an "alternative overnight stay", so to say...

But it didn't answer mine (or Richard's) did it? You plan to get a second residence. I understand that absolutely perfectly. If you manage to do this then you'll have to go and register the fact that you have a second residence. Whether it is the primary or the secondary residence, or whatever order you do it in, you'll still have to do it.

If your permit has written on the back of it that taking a residence outside your canton is not permitted, then I think the answer to this question is fairly simple? One, two, three, residences - it doesn't matter. If it's outside your canton, and your permit forbids it, then it is not allowed. It doesn't matter if you are staying there 1 night a week - if you register it as another residence, then that's how it works.

So, I guess I'll ask again - what is written on the back of your permit? Does it contain some wording that might lead you to believe that it will be ok?

That said however - if you have no intention to save taxes, then you also have the option not to register the fact that you have a second residence. This would of course be illegal, and I would never condone such behaviour

You'll also see that on the back of your permit will be written something like "without approval". If you still plan to pay your taxes in your home canton then you should have the option to go and talk to the authorities and ask permission to get a second residence in Zurich, as long as you swear to pay your taxes in Zug. I can't imagine this would be a big deal for them - I only wanted to point out that you have to work within the framework of what you are officially allowed to do, and this is written on your permit.

ooook, now it's much clearer what you mean... well, my permit says the following: "gilt nur fuer den eingetragenen Zweck und nur fuer den Kanton..." and the "Zweck" is my current employment, then further it says: "beim Wegzug in einen andern Kanton oder ins Ausland hat sich der Auslaender abzumelden" which I assume is not prohibiting the "Wegzug" itself, is it?
I am certainly not planning to change my place of work, so der Zweck is safe here with me... what about Wegzug?

In the case you have the answer is quite simply no you cannot do it without actually getting the written permission from both Kantons - which to be blunt is not going to happen. The whole of this thing actually does centre around tax as Mark alluded to and the Swiss wanting to know where foreigners are. It was not that long ago that you could not, as a foreigner, own or rent two apartments...

As a B permit holder you would have a (slim) chance but an L is (still) seen as short term and for people who are here to work and then go back home to which ever country they came from.

As I recall from the way mine was worded it specifically forbade living outside the canton without permission. I can't see the whole text of your permit, so I can't comment on any vagueness that may exist in the definition (though I would say that when it comes to saying what you can't do, these things are often very clear). I can only repeat what I said earlier though - take your permit to your local community office and ask them your question. If any permission has to be granted it will have to start with them, so you'll have to talk to them anyway.