James has good points. If you are looking for expansion capital for your current capital, adding another will add some complication. If you have used outside investors your ability to offer a separate class of shares for the 2 founders may be limited by what is in the company agreement, operational bylaws, etc. At the least adding more shareholders will dilute whoever has put money in up to this point, unless someone has anti-dilution provisions..which is not likely.
If you can't come up with a real valuation, and there are probably half a dozen models that angels look at for early stage companies, then I would suggest you structure an earn out based on revenues of the new joint company. This is kind of like revenue financing but it adds risk on the other two founders since I assume they would have not real power in the new company, more of a transitional consulting role?
If the two founders want a pay day right away, and the current investors don't want dilution, then perhaps you could bear the brunt of dilution personally (gasp) so the outside investors are only being impacted minimally. If that is the route I would propose some additional incentives as part of your compensation package that are triggered by reaching a performance metric, like revenues, margins, customer base growth, etc. The point here is to get everyone on the same side of the table without creating major strain.
Of course there is also debt (if you have collateral, including A/R or other assets) or you could simply say no thanks and concentrate on your present investment needs to grow your own baby without putting too much financial weight and/or on it's shoulders.