google "wohnförderkonto example calculation finanztip"
In brief. The 2% interest only starts when a withdrawal of the subsidized capital begins, i.e., (and then on the entire accumulated amount). In other words, you save into Wohn-Riester, then when the allocation occurs and you use the equity and loan, the Wohnförderkonto starts. Additionally, if the repayment is also Riester-subsidized, these also flow into the Wohnförderkonto.
Of course, the Wohnförderkonto then plays a role in the tax calculation in retirement to calculate the tax burden, and not as the "whole" amount.
You also cannot simply compare the 2% with the mortgage loan interest rates.
For us, it will be worthwhile (of course, you should not simply spend the tax advantage, but use it for special repayments or reserves). However, we have now calculated a model with our bank where we will not realize the withdrawal now, but will transfer the capital accumulated so far into a Wohn-Riester/TA construct, in which the allocation will take place in about 16 years. Then we will be 52. From then on, the Wohnförderkonto will be "activated"...
For us personally, it makes sense. (the current tax advantage is just under 1000).