Prune only when dormant or not in a growth spurt if you are not JM experinced I'd suggest waiting til fall after leaf fall then you'd be ok ...the only way I know to "make a multi trunked tree is to butcher the thing doen to a nubbin and let it grow back wit a bunch of new brancheds that would beconme trunks ...I do not suggest doing that ...the form may be natural to your seedling but My insionct is that it its a fertilization prioblem and possibly too much shade... I am NOT a fertilizer expert but if others agree maybe someone would have an idea ... My other instinct is to tell you to plant that sucka out get it out of the pot pronto and plant it somewhere it get a little more sun and it may grow to a less "santa" form...David

Well it isn't necessarily unnatural for JMs to become multitrunked to an extent. There are several vase type cultivars (ie 'seiryu') that tend to send out multiple strong limbs that become trunks. The loss of the terminal bud often causes multitrunked behavior...and it is highly dependent on the genetics as to whether it will do this.

Having said that...fertilizer and poor growing conditions could cause this as well.

I personally like to have a combination of the two...multitrunked and single leader trees (depending on the variety). However, I think too much multitrunking can look bad.