I have 3 pair of angels....2 sets bought as adults and 1 set bought as juvies...none are in a tank together tho..and the way i pick my angels (although im much better now at sexing in the store) is i watch the tank for a good 15-20 mins...angels form bonds from juvies to adults and if you watch that tank for a good amount of time, you can pick 2 that seem inseparable..also, the smaller angels you get, seems harder to keep alive...they are one fish that seems to be super sensitive to different
pH's in my experience...its a must to test the bag water you bring them home in and compare it to the ph in your tank...
acclimate accordingly...but you can , just get 2, and have great sucess with 2, if its something you really want.....I did want them badly and took the time needed so I wouldnt have to return any or deal with the heartbreak of them passing ..goodluck in whatever you decide!!
ps..that tank is perfect for a pair!