How large or old are the fry that are already in the tank?
Sometimes larger fry can be quite brutal to newly spat fry which are smaller than them. New 1cm fry with 1" fry may not go well.
If they're within 2 or 3 weeks or maybe even a month older than the 'fry to be', they should be ok.
Typically the maternal instinct of the 2nd mouthbrooding mother will be extended toward the fry in the tank.
Regarding keeping two hold females it depends on the species (for example, two female demasoni's won't work in the same maternity tank).