Workbench can sustain more than one solver, but your licence may not: Research is bought by the "task" so you may only have one solver available at once.
In the below, the "Share a single licence..." is on by default. I usually (when I remember, I don't use Workbench much) turn it off. It's less efficient with the licences but I have several tasks of everything: a perk of being staff!
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