My two rainbows (boeseman's and Australian) have done fine together but just now are swimming very cloe and flashing their fins every now and then and swimming at each other sometimes, they aren't constantly attacking each other just every once in a while they act like that, any ideas?

My guess is pre-spawn. Are they locking lips?

I know the boeseman's is a male but I didn't know about the Australian, can they actually mate?

I've read that barren tanks can make them fight.... do you have plenty of plants for them? I'm also wondering if they need to be in groups so spread out aggression?

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20g Heavily Planted http://www.aquariumforum.com/gallery...00&ppuser=8981
36g Bow Front Heavily Planted http://www.aquariumforum.com/gallery...00&ppuser=8981

It's well planted with hiding spots and everything, they just don't look aggressive towards each other, they face opposite directions and swim close and kind of not nip but open their mouths at one another

Weird. Maybe try getting them each more members of their own species (if you have the room) and see what happens.

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20g Heavily Planted http://www.aquariumforum.com/gallery...00&ppuser=8981
36g Bow Front Heavily Planted http://www.aquariumforum.com/gallery...00&ppuser=8981

Spawning or fighting? You said there was plenty of hiding spots so all I can really suggest to you is to keep a close eye on them, and of they start nipping at each other, seperate them. Hope I helped

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29 Gallon Tank
1 Betta
2 Peppered Cory Catfish
3 Glow Light Tetras
3 Jumbo Neon Tetras
3 Rasboras

They just don't show any signs of aggression, it's more like swimming close and bumping each other. And just today they have been doing the same thing towards my play every now and then. Can the two different rainbows actually spawn?

for what it is worth if they are not showing aggresion I would just watch and see what happens. My senseless betta wanders the top of my 55G while loaches and cories have a blast in the basement...eveyr now and then he must get bored and goes on a mission to chase the cories. they tend to snicker and move on.... he never touches them just tries to be tough guy. who knows what goes on in their little minds. The key is real aggression. But then again the betta is in a freshwater ocean compared to most bettas. He loves it. And I have two juvenile clown loaches ...yes I know one day they need to go to a larger tank or become free agents but they will go to the top now and again under some aquarium sponges i use to slow the flow of my four bio wheel outlets for the beta but he never ever even gives them the time of day and the loaches are always preoccupied with well.... being loaches.

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Hoopman
Sunny Orlando
55G w/Angel, loaches/cats, tetras, rasboras, Two Bio Wheel 350's, Dual T5 ATI bulbs/blue & blue-plus (white), give a saltwater tank look/moons. - 20GL w/Angel, Mystery snails, tetras/cherry barbs. Dual 24 inch T5 with same ATI bulbs/moons. Dual HOB filters.

You're seeing textbook dominance/aggression display. The first phase of a fight among young human males is trash talk and chest beating, and that's where they are. They slide up beside each other and push water at each other, showing their strength. the loser will generally turn down his colors, til he tries to dislodge the alpha again.
Cichlids are famous for these displays, but most fish do this.
It can move to fighting, but it doesn't have to. Rainbows are social fish, and one of a species of shoaling fish is always a bad idea. They don't know what to do with themselves and they can become aggressive.
If they are different sexes, then the male will try to show his fitness with a 'lateral display' like this. They will cross as they are both in the Genus [i]Melanotaenia[i]. Hybrids are a real problem in the rainbow hobby, but since the fry are murder to raise, it isn't anything for the non-breeder aquarist to worry about.