Re: Rome I kind of siege by multi number AI controled armies.
Never came across anything in that aspect - it would appear that allied co-besiegers are considered re-enforcements in M2TW, not besiegers themselves, hence do not build equipment.
To be honest - in RTW that meant certain doom for the besieged if highly outnumbered while in M2TW you actually have a chance. After M2TW they didn't bother with ladders much and in S2 itself had the infantry nijna up the walls. I rather stick with M2TW where this issue is concerned.