You will have to do the math to determine amount of undercut required from the
smaller feature but you could consider doing a two photoresist step process. Do
your deep etch first and then align the second groove to etch into the deeper
groove during a second photo/etch step. Since the first groove will be filled
with photoresist as you break through laterally it should be a nice channel.
Alignment will be a little tricky but you should be able to accomplish what you
are trying to do. I know that we could do it using dry etch techniques within
our foundry so if you are interested contact me directly. Bob Henderson