You must decide is it worth destroying one of those stacks totally and moving your army there, to take the hitting from german 2nd stack or you could again try doing hit and run and keeping your center really strong.
If the soviets manage to succeed really well in it’s first attack and destroys all inf + art and maybe some tanks. The soviet tactic really on inflicting a lot of damage on it’s first real test and that is when the germans decide to start their push to moscow. The belorussian stack tactic is really good, but soviets must gather offensive power in their first 2 rounds. So that they can attack, inf only is good for getting hits, but you really don’t do damage in defence that well and that is why the soviets must attack first.
Belorussia is the center where you always get the chance to attack first. If germans are building their own 1 stack army tactic, you must from the very first turns begin preparing for it. So offensive power, more and more for soviets. Inf only isn’t the only option for soviets, but you must play wisely and defensively. And USA and UK must help with moving more and more planes to your side, the germans will crack like a nut, when soviets starts to have 3 Bombers 6+ fighters. Their offensive power after that is almost equal to Germans + they have the advantage on defence keeping their fighters on offensive and in defence to maximum effect.