The issue is one outlet has slightly more restriction than the othe.
A Y joiner you most likely already have ?
Using two in line valves will mean you can reduce the flow to one filter. While aquarium shops sell tapes/ valves it’s common to use 4 mm irrigation fittings from a hardware store.
You have to use one of the gang valves that has a separate control for each sponge filter. Otherwise, you will never get a balance of air flow between the two filters. If you just connect the two using a tee connector, as soon as one filter gets enough air, it will take it all and the other filter will never get any. It does not matter whether you are using an airstone inside each sponge filter or just an open tube. You still need a gang valve.
I am a fan of the two-way gang valve shown below as it contains a built-in check valve. I use one on a tank with two internal filters and it works perfectly.