For the very best quality (ie. line width, resist adhesion), I would
recommend using different exposure energy for 1.3 mil and 1.8 mil resist.
I would also suggest that development speed be adjusted somewhat slower for
1.8 mil vs 1.3.  Running them at the same exposure energy will risk
underexposure on 1.8 mil, which could result in lifting resist at develop and
etch.   The same is true with developing.  1.8 mil resist should develop
slower (allowing a longer dwell time in developing chamber) than 1.3 mil.
Running them at the same speed will risk slight underdeveloping, (leaving a
resist "foot", or worse, scum) which could cause shorting at etch.
      Your resist supplier can help you set up the ideal parameters.  While
one of the above conditions alone may not cause any problems (depending on the
application), together they could be a source of major headaches.