In short: use some smaller ships as escorts to let your capitals vent and not get swarmed!
Capitals ships are great at being concentrated sources of BAM and great duellists/tanks, but like all ships they just don't do that great when they are swarmed (at least not without some specific builds). Like, paradoxically, your 2 capital ships would probably win if the enemy was all capital ships themselves! Its those pesky small ships constantly firing and stopping any venting (and getting up the Legion's engines) that are the problem.
Lets see, a paragon and a Legion is 100 DP, so (if your battle size slider in options is maxed to 400) you have a budget of 60 more points without any capture points. 3 more destroyers (plus your sunder for 4) and 4 frigates, officered or not it doesn't really matter for these fights you just want some numbers and distraction, takes your ship count from 3 to 10: much better for letting your capitals do their thing and pop enemies.
If the logistics of that many more ships is a problem, then I'd say its better to leave one of the capitals at home for a bit as they are very fuel/supply expensive, especially legions and onslaughts (my poor favorite ship is a hog).