To chime in:
- you can not split an existing 3a account
- not all Kantons accept any number of 3a accounts, you should first ask the tax office. AFAIK two accounts are accepted everywhere
- many banks open no more than two accounts for the same customer
- deposit insurance is limited to 100k per customer. Most "Kantonalbank" are fully guaranteed by their Kanton (i.e. not limited to 100k insurance), the exceptions being Vaud, Genf, Bern
- transferring a 3a account is free if it's cash-only
- transferring an account that also contains funds will probably cost, and may not be possible to begin with - ask the receiving bank if they accept your funds at all. Selling and buying back at the new bank will also come at probably not-insignificant cost. Whichever way you decide, try to have the new bank shoulder (part of) the transfer cost (they'll be making money on your 3a after all)
- each bank offers different funds, usually only their own
- buying and selling the funds will come at a cost, factor that into TER over the period you expect to hold them
If you intend to (mostly) invest in funds, IMHO the most important factors are their funds' typical TER rate along with past fund performance. The TER will most probably have you strike CS and UBS from your list of contending banks. If OTOH you intend to have your money in CHF the most important factor is the interest rate.
Since there's a clear timeframe during which you can(must) withdraw the funds at retirement, there's a limit to how many accounts are useful.
As for calculation taxes due upon withdrawal (both in english): CS calculator Postfinance calculator