Yes. You can connect them is series (one after the other) or in parallel (inlets and outlets connected).
If they are in series you get twice the storage but there is a delay in the second heater coming on so you don't get quite as high a recovery rate.
If they are in parallel then the second will come on sooner but you have the problem of balancing the flow.
Since you should have isolation valves on both heaters, you can use them for balancing the flow. The way to do it is as follows.
1. Set both heaters to the same operating temperature.
2. Starting when both heaters are off (water is hot), and all water heater valves fully open, start filling your tub. Note which heater comes on first and throttle that one just a little bit.
3. Repeat next time you fill the tub. Working with the same valve, throttle it more or less to try to get the heaters to come on about the same time.
Don't try to get too precise because it can't be made to come out exactly the same all the time. Your goal is to make sure both heaters come on while you are filling the tub.
If you aren't sure what you want, starting from a parallel system with isolation valves on both heaters, it takes only two more tees and one more valve to be able to switch between parallel and series.
If you have a parallel setup it is easier to maintain hot water when you have to take one heater out for service or replacement.