Signal levels while doubling

I've been playing full time double bass and bass guitar, but have only recently started doubling. Actually on db and ubass.

On 90% of my gigs I do not get the opportunity for a sound check. As a result I find it very challenging to get both instruments to sound at the same volume. The fact that the double bass has a significant acoustic volume makes it very difficult to judge when setting up, especially when I don't know how loud the amp is going to have to be.

I use a radial engineering acoustic preamp pedal to switch sounds.

Any pearls of wisdom?