Welcome to the Mattress Underground and I would be happy to lend my over 20 years of mattress design and consulting experience to help you better understand what I think will work best for your situation, which is not that uncommon, but still a bit more challenging. First, I can understand why you would want to consider a split mattress with his weight being much more substantial than your weight, but it may not be as necessary as you think, since you are also having back and neck issues with the current mattress that has lost the necessary support for your body, not just his. That certainly points to the fact that likely both require a more supportive mattress that will also be more durable than your current one. While I am not sure what mattress you purchased, with both you and your husband having back issues and being side sleepers, his larger frame will still require his shoulders to sink enough into the mattress, which is also the same plushness that you will need for that extra pressure relief. However, for you to both get the low back support you need, the mattress also has to be firm enough to provide the necessary support in the mid-section. That may or my not have been how your current mattress started out, but either way, it was certainly not what you ended up with in far too short of a time period.
As a Doctor of Chiropractic, I can tell you that staying in proper alignment is absolutely key to back and neck issue, but I realize that it is not so simple as to find a firmer mattress, since as people, we tend to be heavier in the mid-section, while being lighter, yet broader more often in our shoulder regions. With the Back Science mattress, I helped create a special interlocking specially zoned Lumbar-Flex support and I have been able to test and verify that it really does work to produce the most optimal alignment thanks to hundreds of customers that I have measured using the Mattress Checker sleep posture App. In your case, I would recommend the Back Science Series 2 Hybrid mattress, since it should work perfect for both of you needs as a luxury firm design that will support the back, while using a special hypergel that is not only cooler, thanks to our patented Airflow Transfer System that makes it up to a 100 times more breathable, while offering full contouring and comfort for the shoulders and hips. It uses an over 1000 count individually wrapped coil system for a more graduated support that is also extremely low in motion transfer between partners, with additional Quantum edge support.
In a recent survey we had Trust Pilot run found that 97% of our Back Science mattress customers felt that the Back Science mattress reduced their back pain, but it also is very pressure relieving and comfortable, so you should not need an extra topper to give you the pressure relief you are looking for. The Back Science mattresses all come with a 365 Night comfort guarantee and while I doubt you will need a split firmness that is different from your husband’s, in the event the mattress is somehow to firm or soft for either of your needs, we offer completely free customizations. So, you don’t have to just discard the mattress if it does not work the first time and it is a very easy thing for us to modify thanks to our GripLock modular cover system that uses a zipper to allow easy access to change the internal comfort and support layers when needed and it is also done through simple ground shipping right to your home. As far as durability goes, our Back Science Hybrid series mattresses all come with an industry leading 30-year limited warranty that kicks in with only ¾ of an inch of sag, as compared to the industry standard of 1.5 inches with most brands, which should give you far more assurance that it will stand the test of time.
I hope that helps, but feel free to let me know if you have any additional questions whatsoever and if you do purchase, be sure to use the TMU10 discount to save an additional 10 percent as a Mattress Underground member.