If you catch the remaining one and place your finger on the vent you should feel the end of two little bones. If there is half inch or so space between them you have a sure hen. Cocks, and hens that are not laying, have the bones close together, almost touching. This is one of the rare occasions when you can sex the lovebirds. With adult birds it is possible to get a fair idea of the gender by catching and letting the bird bite your finger, the girls always bite harder than the boys, you need some experience to compare though.
They don't fret for long but. If you add another bird now, the present one will almost certainly kill it, no matter what the gender is - you need to give it a week or so to settle down. They are vicious little brutes, dispite their name. The easiest way of introduction to one in a cage is to put the new one in another cage next to it. Some take to their new partner very quick, other are more fussy. You'll see by their behaviour how suited they are.
In an aviary it is ok to add a new one straight away if there is enough room for them to get out of each other way. Never put three adult lovebirds together, the odd one will not survive long. In a roomy aviery you can put three or more 'pairs' together but two boxes for each pair should be provided.