Hi brooklyn22,
Welcome to the Mattress Forum! 
You can find the specifications of the Brooklyn Bedding Best Mattress Ever in the simplified choice thread here. It doesn’t use a separate topper, but instead varies the ILD of the upper latex layers. The medium uses 2" of 28 ILD Talalay over 2" of 32 ILD Dunlop latex, and the firm uses 2" of 32 ILD Talalay over 2" of 34 ILD Dunlop latex. The polyfoam core I the same 6" 2 lb density. There is a small amount of polyfoam in the quilt that acts as a comfort barrier between the latex and the sleeper for a slight change of the surface feel of the mattress.
You can read my thoughts about buying a mattress/topper combination that you can’t test in person in post #2 here. I generally advise to avoid buying a firmer mattress and then adding a separate topper afterwards unless there is no other reasonable alternative or unless you have a chance to do some careful and objective testing for PPP (Posture and alignment, Pressure relief, and Personal preferences) on the specific combination in person using the testing guidelines in the mattress shopping tutorial. Deciding on a topper that would be a good match for both you and the mattress (the specifics of the mattress can make a significant difference in which topper would work best for you) can be almost as challenging as buying a new mattress in the first place. There are specific instances where this combination can be preferable, and you can always take a mattress that is firmer and make it a bit softer through the use of a topper, but it can be difficult knowing if this would work “out of the box” without some specific testing in person.
Latex tends to be a bit more point elastic, so you should expect less of the roll together on an item like the BME. You’d also want to double check your frame support network and foundation (if you have one) to make sure that there isn’t any sag in that are that may be contributing to the two of you rolling together.
You didn’t mention how large your husband is (only that he is substantially heavier than you are), but if he is of a higher BMI it can present special challenges that generally require firmer materials (in the support layers especially). The same overall guidelines apply with higher weights though that PPP (Posture and alignment, Pressure relief, and Personal preferences) along with using high quality durable materials that will maintain their feel and performance for longer periods of time are the way to make the best choices. Heavier people in general will need firmer and thicker comfort layers and firmer support layers than those who are lighter, which is why I can see that you are contemplating a firmer base mattress with an additional topper above that. Post #3 here has more information and suggestions about heavier weights that is worth reading.
With your husband’s bad back, there is no one firmness level that is “best for backs” in general because it would depend on the body type, sleeping style, and individual preferences of the person. I would tell you to make sure to defer first toward alignment, having deeper layers that are “firm enough” to stop the heavier parts of the body from sinking down too far and make sure you sleep with your spine and joints in good alignment and upper layers that are thick and soft enough to “allow” any pressure points such as the hips and shoulders to sink down far enough to relieve pressure points in all your sleeping positions as well, but I know that “firm enough” and “soft enough” can vary widely from person to person.
Another key thing for you to consider would be to become completely familiar with the return/exchange policy of any company you are considering, just in case things don’t turn out as well as you might expect.
I understand your concerns about your husband and his bad back, and if choosing between surface plushness and deep support, I would defer toward that support and alignment, based upon sleep ergonomic research. Unfortunately, I wouldn’t be able to tell you if the medium BME would feel “supportive” enough for your husband (I can only comment about the “how” of choosing a mattress), so your thoughts of going firmer with the potential to add a topper down the road does have logic behind it, just as long as you realize that it does introduce extra variables into your mattress-shopping equation.
I’ll be interested in learning about anything you decide to do, or of course any other specific questions that you may have.
Phoenix