the obvious ones are Lowepro's own sized according to the lens that you want to carry.I have a Lowpro 75. Every now and then I want to take 2 lenses out. What case/bag for one lens 100% safe I can add onto it?
Depends which 2 lenses you want to take. Pretty sure my toploader's a 75 and it comfortably holds my D700 with either a 12-24 Sigma or a Sigma 8mm fisheye fitted and the other in the bottom of the bag. Since you have the bag, try the combinations you want to carry. IIRC, the toploader has the loops to attach their lens cases to the outside so I would use one of them if I wanted to carry an extra lens on the toploader, although TBH, I would generally use a different bag.
the obvious ones are Lowepro's own sized according to the lens that you want to carry.
I also have a water bottle pouch on the other side.
IThat's what I'm looking for but admit I don't know what Lowepro's ad on's are called or where to find them.

I think they are called slip cases, if they are what I am thinking of.
like the ones on the side of this LP Off Trail 2
Light mode - Lowepro Off Trail 2 by Synchroni, on Flickr
I have one of these, and they are brilliant and comfortable, I used to carry my old d200 and 80-400vr and use the side pouches for water bottles on an allday jolly at Goodwood.
I have since upgraded to a Slingshot as i can carry a few more bits and bobs!
hope that helps!