Hi,
I am certain that you are already aware, that a VLAN is the segmentation of a broadcast domain.
IP itself is a broadcast technology.
As such, any device which send a broadcast may inadvertantly end up reaching the wrong destination.
With this Config / Design you will troubleshoot the hell out of any Device (not only Juniper) and never reach your Goal because this Idea is broken by Design... You should avoid that and only use one subnet per VLAN.
Try google for "2 Subnets in a single VLAN" and you will see, that you don't want this - regardless of the Vendor 