There are three ways to make it work:
1)Choose amps whose gains closely match.
2)Choose the higher gain amp such that it includes a gain control.
3)Insert a suitably chosen passive attenuator in series with the input to the higher gain amp, as Newbee suggested.
Be aware that if the solid state amp has a considerably greater power rating than the tube amp (or vice versa, although that would be unlikely), you will not be able to utilize a lot of the power capability of the higher powered amp. How much of the power capability of the higher powered amp that can be utilized will be limited by the clipping point of the lower powered amp, if you do not use an electronic crossover ahead of the amps.
There are a lot of prior threads about passive biamping. After reading them, you may conclude that the added complexity, expense, and potential pitfalls that it presents are often not worthwhile.
Regards,
-- Al