This it not the first time such a suggestion was experessed.
See this post in "Send WUs to 3 users instead of 2?" for example.
As PB pointed, it would need rewritting a lot of aplication and BOINC server code, quite complex.
And potentional gain is questionable.
Another approach is to make LLR running on GPU. This has been tested but benefit was not there - it wasn't consdiderable fast and still used CPU code to feed GPU.
Yet another approach is to make LLR application (that does SoB test) multi-threaded.
A single test would run shorted time - 1/2, 1/4 or even less time depending on number of cores and overall effeciency of application.
But this would also need a considerable rewrite of LLR app, extensive testing and some adjustemnt to the BOINC server side.
Above all, PrimeGrid project is not author of LLR but rather a contributor and intensive user.