After Lola is feeling better and has imprinted on birch, I would keep the sessions separate for the time being. Do one with truffle odor, one with birch. Or one day work one scent, next day work the other.
Eventually you will be able to hide both scents at once in the same environment and expect her to alert on both. I would also keep the boxes you hide them in separate for the moment to avoid contamination. Since she already knows the truffle odor you don’t want the lingering odor to be present on her “Birch” box, and have that be the reason she alerts on it.