There are two problems involved in running an _expression_ pedal through an A/B switch.
First, the _expression_ pedal relies on a TRS connection while most A/B stompboxes make only the TS connection.
Second is the question of what happens when you switch away from one of your targets: it's like unplugging the _expression_ pedal. You might like to have the "last" pedal setting maintained when you have the pedal connected to a different target.
The Mission Engineering Expressionator is designed to control up to three targets from one _expression_ pedal. Expensive, but it's the only game in town, AFAICT...