For reference, there are a couple options for future reference.
1. The dounut suggestion - You could attach/glue and acrylic spacer/riser to get one higher than the other.
2. Notch one/both bulkheads - The mechanical seal shouldn't have to be the full 3/4" that they are, you can sometime get away with trimming it down.
3. Specialty bulkheads - You can buy soem bulkheads that are much thinner and require less space. They are a bit more expensive, but they could work.