The design I was referring to is typically only sold by companies that have a good working knowledge of the local water conditions. This picture shows the general design, a single valve regenerates two tanks with the same regenerant. it is a highly efficient design but does take some knowledge to do properly. my own system has 4 tanks regenerated by 1 control valve. The picture below shows the general design, for your application, the first tank under the valve would be water softening resin, the second tank would have your tannin selective resin. For that application, the second tank would be a nerrower tank so the backwash rates would match. you would calculate the capacity to the tank that would run out of capacity first and regenerate the system at that point. It is a great design, but has its limitations. A res-up feeder and occassional resin cleaning should be done for technical reasons. If your tank was a little larger, you could even add the tannin selective resin to the softener tank, but I am not a fan of this, too many problems.
If you do add the Tannin 2500 system to your current equipment, be sure it goes after the softener.