If you have a Reserve card and add your wife as an AU/Additional Cardholder to that same account with a DL Plat card (no additional AF, no SC access for her), her spend will appear on your Reserve bill and count toward the MilesBoost spend thresholds.

This is what Mrs. Lee and I do.

Note that most benefits/RDM/etc. will go to your Sky Miles account (MilesBoost RDM/MQM are an exception since you can gift them to anyone). She won't get the free checked bag, priority boarding, etc. unless flying on the same PNR as you, though she will be able to use her DL Plat Amex for discounted entry to the SC.