Balancing flow can be a very tricky proposition. If you enter at a given location and eventually get back together at another location, then you have a defined pressure drop between those two points. This means that each of multiple paths will have the same pressure drop. The flow through each path is then a function of that pressure drop.
If the resistance of the paths is not uniform, then the flow will also vary. The variance may be minor (all within say 10%) or major (one of more branches with essentially no flow). Here's the rub: If the inherent resistance isn't pretty much the same, then usually the only way to equalize flow is to raise the resistance of all paths to match the highest resistance. Obviously this drops overall flow rate (and convection). Lowering all resistances to match the lowest or even modifying them to meet the "average" generally is not an option.
You may find that the inherent resistances are close enough to ignore their differences. If this isn't the case, things can get nasty in a hurry. This gets doubly-ugly (is that an acceptable term?) when you consider that you probably want to have a decent pressure drop at the entrance to get a good impingement velocity.
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