The Avatar Parameter Driver component can add to a paramater, but cannot add two parameters together.

The only way to do this is to use an Animated Animator Parameter, which cannot be sync'd. To sync the result of this you need to use proximity contacts and sync the result. This whole system means that to do basic maths operations, you need to have multiple layers, contacts and excess parameters. In general, this is a very clunky system, which creates unnessecary lag.

The Avatar Parameter Driver can already add a fixed value to a parameter, allowing it to add two Parameters together would cause a lot less friction in these cases.