I do this every time. I pay the road tax on the Motability car (usually low and hopefully nil next time) and claim the exemption on the used car (usually sporty and big engined (sorry but I’m car mad.))
The key is to be clear from the start when ordering and definitely when finalising collection. Dealers will normally assume you will use your exemption on the Motab car and register it as disabled class. MAKE SURE you remind them not to do that and to register it PLG (standard car class.) Even after reminding, I’ve had dealers register Motab cars as disabled so once ended up having to sort it myself.
Hope this helps?