Aside from having them DNA'd, I would definatly have a second cage ready. Because they are notorious for fighting and killing, I wouldn't want to take the chance. What you are describing sounds similar to 'sibling rivalry'. I had several clutches of lovies and saw what sounds to be exactly what you are experiencing. They will be perfectly happy together- feeding, playing, etc andthe next minute flapping and flighting. What really helped mine to get over the fighting stage was seperating them, but keeping their cages niext to each other. The bowls on the same side, perches, etc... That way, they can 'eat' together and 'perch' together but you not have to worry about fight/kill. Later, you can let them out to play.

Males tend to get along fine, though when they reach 'puberty', they can get territorial (i.e not letting the other eat, guarding territory) and this can lead to the death of the 'weaker' or less strong willed one. My friend had 2 male lovies (PF I think). She was told they would get along fine, etc...One day (after having them for about 3 1/2 yrs) she was home sick and heard screatching from the cage. The dominant male was attacking the other. She was fortunate enough to save him. Interestingly, they get along great outside of the cage.

Avian Biotech will sex them for $20 apeice (what we used). Its not always 100%, but pretty close.