OK assuming that they are 50 / 50 owners, and someone actually paid for all of the mortgage interest ($25k + $15k = $40k. ) Also you state that mortgage principal is both $2M and $1.6M so there is that.

Divide the interest into qualified residence interest and excess interest.  The QRI will be $750k / $1M * $25k for each the BF and GF = $18750.  The QRI will then be allocated between the rental portion and personal portion.  So QRI schedule A - $16,875 and QRI schedule E $1,875.

The excess interest allocated to the rental property will be deductible on the schedule E - so 10% of the $6,250 or $625 will go to each of their schedule E - for a total schedule E interest of $2,500.

You seem to imply that they paid different amounts of interest - $25k + $15k = $40k.  So I don't know which part is off, but I would probably ignore it.