As long as you arrange things first with the other player in an OOC fashion, there's no reason why you can't RP pickpocketing. However, to show the other person courtesy you two should agree in advance on what the roll and counter roll should be, and what if any item could be lifted. This should not, however, be used as an excuse to mule or transfer rare or valuable items, and a good dose of common sense and maturity is needed.
If both players have the maturity to handle the situation and can arrange the mechanics or RP themselves, or there is a GM to moderate it, yes, you can pickpocket other PCs. Similarly, you could use this on a quest with the GM's permission, or they may ask for the roll.
Hope this answers your question.
