OK to use plywood instead of hardieplank between two close windows?
Yes, but I’d use an exterior plywood product made for that type of application, like Medium Density Overlay (MDO) plywood and I’d be sure to pre-prime the edges. That is to say, I’d paint primer on the edges before installation. 

In order to seal the window edges and building paper, I’d use a peel and stick seal tape. 

Jack is correct that trim is customary for spaces up to about 11”.