No, it's not possible to get a single public seed for all future addresses.
Let's say you have three private seeds,
Sc with public seeds (xpubs)
Pc that generate the addresses
Then, the way to calculate the multi-sig addresses for a 2-of-3 multisig for these three seeds using P2SH is the following:
You create a P2SH redeem script that looks as follows:
OP_2 [Pa1] [Pb1] [Pc1] OP_3 OP_CHECKMULTISIG
Then, the "multisig address" is the RIPEMD-160 hash of that script.
Thus, to know the next address, you will need to calculate a new RIPEMD-160 hash with the script now containing
Pc2. There is not a multi-seed of any kind.