I'm curious why you chose to buy those particular varieties having no information about them.
Elstar apparenly flowers in mid May according to Brogsdale (in England). The describe the fruit as sweet, crisp and juicy. Ripening in early October and keeping (natural storage) October-December. That doesn't sound very long to me, but I'm thinking of apples like Fuji and Honeycrisp.
Vlaanderens Roem Rode Boskoop "Fruits have soft, fine, creamy white flesh with a sweet, subacid flavour." It finishes blooming a few days after Elstar begins. There isn't too much overlap but if those were the only two trees in the area there would probably be some cross-pollination. Ripening and picking dates seem to line up with Elstar.
The brogdale site has pictures of each.
If you can use Google type "elstar apple" (without the quotes) and the very first link has plenty of good information. http://www.orangepippin.com/elstar.htm
Rode Boskoop brings up plenty of links in Google, but they aren't very useful if you don't speak Dutch. One of them indicates that it is the same as "Boskoop Rouge", for which there are English language hits. This is one http://pepinieres-arbres-fruitiers-form ... ieteUK.htm