As the original owner of a 1996 26X I'll confirm there are 2 valves plus the vent. In my opinion, the two valves are needed as it seems to take a long time to fill the ballast using just the transom valve along with the vent. I've only had to replace the plastic handle on the rear valve otherwise all are still in good shape. Check by opening the transom valve on a hot day to see if air pressure has built up in the ballast. You will hear the noise from the ballast de-pressurizing an indication there are not major leaks in the ballast. Do recommend inspecting both valves on receipt.
For what it is worth I have a 26X, hull#10 and it has only one fill/drain valve. The air vent is up in front under the V berth. That location is not very accessible and the newer mod would be much easier to access.
A number of years ago I replaced the fill/drain valve with one from an RV parts shop and it works quite well.
Another improvement I made was to get rid of the weak aluminum rudder brackets and replace them with DIY stainless steel brackets that are much stronger.
My buddies 26X is only 50 serial numbers different than mine. His vent valve is under the v berth which usually gets buried with stuff while mine is always accessible under the cabin step.
His ladder is outside the rudder, allowing for a wider and bigger motor but with shorter legs my girlfriend does not like its entry having to straddle the rudder to get in.