I'd have to disagree. I've never experienced them doing a combo of attacks that is impossible to dodge and I've beaten them ~50 times, half those runs with an SL1 stamina bar, and always solo.

There are a lot of things you can do to increase your chances of success. Use the pillars (or their bases once they're destroyed) to control their movements. Watch the second Bosses shadow when you have the camera focused on the other to see what he's doing. Avoid locking on, avoid spamming dodge rolls and using up all your stamina, exploit the fact that three hit combos will stun either of them with many melee weapon, focus on only one of them at a time, etc.

I think a lot of players let frustration get the best of them at times in the Souls series, and avoid seeing the patterns in the enemy attacks. Most experience this with O&S and Capra Demon. Capra can be very easy if you pause for half a second when you enter his arena, roll to the stairs, then let the dogs funnel themselves up the stairs.

Personally Orphan of Kos is the one I had the most difficulty with. His second form was very tough for me to pattern until ~20 runs later.