I don't really know anything about eventing (did XC once haha), but I came from the world of jumping into dressage. I also know a good handful of eventers who are focusing on dressage now. My advice (for what it's worth)? Work on your dressage first. Make sure you have adjustability within your horses gaits and between gaits, control, good rhythm, cadence etc. Basically all your basic dressage work (think solid 1st level for your entrance XC level..whichever that would be). If you are working a good 1st level you are working at a higher level than the entry level event dressage test from what I hear (may be incorrect there). Even if you just jump, and not event, it will still help with your jumping for sure. You can learn both at the same time yes, but many of the eventers/jumpers I know (this obviously isn't the rule), wish they had started with the dressage. And now that they are in dressage lessons regularly, they are winning their eventing on the dressage scores alone, sometimes scoring an easy 20% ABOVE the competition.
Even if you don't go into eventing, and just jumping, your dressage will only help.
My 2 cents :)