Is It Okay to Connect Two Water Heaters in a Series (so that water from one flows into the other)?
Yes you can do that but it will be highly inefficient.  A single larger tank is more efficient than two smaller ones with the same total capacity.  This is because they have more surface area between them for heat to be lost from.
So install a 50 gal. tank.  You might consider an on-demand/tankless heater for your tub needs.  That way you will have all the hot water you want when you fill it but won't be paying for holding a large quantity of hot water on the chance you might use it.