reason being is Dish is aimed at 110 & 119. DirecTv is 101 (main), 110 & 119.
Also, more often than not, its a Dish Twin and the switches built in the Dish dish won't work with Direct.
Yes you can do this only if your E* oval dish is legacy dual LNB. Identified by the fact that each dual LNB is removable. If an integrated E* multi-switch dish you can still purchase a legacy E* oval dish off Ebay or choose to switch to an Oval-3 integrated dish from D*.
If a legacy E* dish, you will have to order a Sat-C LNB kit for 110 and mount a 2nd 18" dish pointed at 101. This configuration actually yields better peaks than a D* Oval-3. The only problem is you must run five RG-6 cables from your roof and I recommend that your multi-switch be located inside the house.