Switching hands between throws is generally acceptable.
Switching hands during a throw (priming with one hand, delivering with the other) is generally not acceptable. In particular, the Official Rules for WRPSS-sanctioned tournaments forbid switching mid-throw:
4.0 Once the fist has reached the highest point of the final throw of the last prime, the delivery of the throw is considered to be "in approach". At any time during the approach of this final prime, the hand may be released in any of the following manners:
However, not all tournaments are run with WRPSS rules or WRPSS-standard refereeing, so switching hands mid-throw can be a useful tactic in those lesser tournaments (particularly when playing against someone familiar with WRPSS-sanctioned rules only). And it's regularly seen in Street RPS circles.