I would probably put the full amount on schedule E on the Rent income line then subtract out 1/2 of the 1099 on line 19 other expenses with a description like "Portion of 1099-Misc belonging to others"

Something like that - just to get to the correct result.

And tell the client to have the payer pay each of them separately next year and issue separate forms 1099.

Of course if all the payments were made out only to your client - I suppose the argument could be made, that all of the income is his, and perhaps he should have issued a 1099-Misc to his partner.

I think I may have just confused myself. lol.