Dan,
I'd be interested to know how you get on, since my EPOS ES30s are also a tri-wire (only) configuration. If I upgrade to decent speaker cable in the future (Townshend Isolda) I'll be getting just one pair, then perhaps a second paid in future so will face a similar dilemma albeit with cabling rather than power.
Suck it and see seems to be a good way of finding out though! But I suspect isolating the bass wold work best since its the most power hungry, but may depend where the drivers cross over. EPOS mids tend to go a little higher up the spectrum than average so isolating the tweeter will mean one amp is powering a good part of the lower spectrum, although you might then find the tweeters are very sweet whilst the bass doesn't quite have the power it might.
David