The split ratio is likely too low to maintain a consistent pressure. Typical split ratios are 10:1 or higher. If the sample only needs to be diluted 3-fold, it can likely simply be diluted before being injected and you can run a splitless injection. Otherwise, use a higher concentration and split at a higher ratio in the GC.
What can also affect this issue is whether you're running in Constant Pressure or Constant Flow mode. Which mode are you running in? Is there a temperature ramp in the method? What's the initial oven temperature?
What o-rings are you using on the inlet liners? I have seen people accidentally use the Flip Top o-rings meant for a specific 6890 inlet weldment on a 6890 or 7890 and the inlet top cannot seal correctly leading to pressure and flow issues.
Also, have you checked the split vent traps in their holders under the cover on the top back of the GC? The inlet flows you're mentioning are pretty high and since it's happening on the front and back inlets it seems like a problem with maybe a consumable recently installed.
I agree with valentinrusu's comment regarding the split ratio; a 2:1 split is low, especially if you have a sampler like a purge and trap in line. Has the system worked before with these conditions and did it just stop working? If the system used to work, what (if anything) was changed before the problem started?
What is the pressure on the gas tank that is supplying the inlets?
I went ahead and flagged the “Correct Answer” on this post. If you still need help though, please let us know and we would be happy to continue working with you.