It is actually a more complicated issue than you might think when you really look at it. Good luck!
Seeing that RMU is still "declarative" I assumed that it is viable. I'm a little bit surprised in reading that it may not work instead. I give an example to try to explain what I mean.
Keeping momentarily aside istantaneous spell and referring to melee (and sticking to RMSS), in my opinion even just the basics of melee are not obvious when paired to the repeatedly stressed (in the rules) need to parry. Parry must be declared at action declaration. Assume you have a +80 broadsword skill. You declare a React & Melee (-10) Action. So you only have 70 OB to split; assume you split it 50OB/20DB. Following the very same example in the RMSS book, it happens that you act in the deliberate phase:
At the beginning of the Deliberate Action Phase, Dral declares his action to move and attack. Since Dral's running movement rate is 100' and he has to cover 30', Dral uses 30% of his activity to reach the Orc. So his attack on the Orc is modified by -30 plus 10 because it is the Deliberate Action Phase. What does it happen to the OB/DB split? Does it mean that it is now 30OB/20DB? And if the split were 0OB/70DB would it still be -20/70?
I would rule that you must keep the proportion (-30*50/70 to OB, -30*20/70 to DB). Or, alternatively, that you must apply the penalty first to the OB, until it goes to zero; then, if there's still a penalty left, you must reduce DB until it goes to zero; then,if there's still a penalty left, you must reduce further the OB (attack) below zero. Or, third method, you split the penalty evenly, provided you do not go below zero with either portion of the split, then you reduce further the other, until both figures are at zero; and then further reducing the OB (attack) portion of the split (below zero), if there's still a penalty left. In any case, if the DB has been already committed to defend, the penalties could be applied only to the OB portion of the split.
I see that in RMU you can cancel a declared action and, say, having lost Phase I, you have left 70% to act. What does it happen to the original OB/DB split? Maybe I've missed it, but I've not seen any suggestion. Am I wrong? As it's due to canceling, in this case I would rule that you can rearrange it as you see fit (your overall OB is at least at -30 now).
I supposed that this kind of things has already been ruled in a way or another. Maybe they should be officially suggested (in RMU, rules can still be updated; for RMSS there are the FAQ somewhere, but I've not seen anything about these issues; maybe I'm wrong, have I missed them?).
Now I'm turning to the suggested threads and I'll let you know what they tell me.
Thank you.