My understanding of taking amphetamine/methamphetamine at the SAME time as MDMA is that the result is higher neuro-toxicity as the seretonin transmitters end up in the dopamine receptors and visa versa, which is REALLY bad for your brain, however it feels amazing! I love MD with Meth (or dex) aswell. That could be an urban myth but i've heard it a few times including on Bluelight so it could be credible - not totally sure
However its irrelevant in your case the Dex will have mostly worn off by the time you drop your MD, so i'd say theres no problem. And yes there may be cross-tolerance in general between the two, but as a one off, with only 10mg dex (a tiny amount), the tolerance wouldn't show. If you were on 30-40mg dex a day and had been for a while, and wanted to take MDMA then I would say yes there will be some tolerance and the speedy (dopaminergic) aspect of the MD might not be quite as strong. Remember though that Amphetamines (like Dex and Meth too) are primarily Dopaminergic which is the UP - chatty - wide eyed - can't stop talking etc, whereas MDMA is primarily Seretonergic which is the loved up, everythings beautiful, touch and empathy, etc. They are working differently.
I can easily chew down 150mg of dex in a night if i choose to and throwing in a pill is magic. Dex IMO is underated as a recreational drug - 100mg dex (20 tablets) is pretty similar in effect to a couple of lines of meth. And the nice thing is you know it is clean, pure, and you know for sure that it will do what it is supposed to do, rather than taking a gamble with an ecstasy tablet or a line of meth.
THats my opinion....