reconnecting extra bathroom to existing sewer system

What's the proper way to connect two 3" sewer lines from the house to one 3" line to the septic? One house line is on the left and one on the right of the septic line and both will need to make a 90 degree turn to connect. On the original install the guy used a clean out wye which constantly caused the left side house line to plug up. The line from the right only services a bathroom in an addition, the line from the left is the main line for the rest of the house. I disconnected the addition bathroom line a while back and just have the other line connected to the septic inlet via a 90 elbow. We're getting ready to remodel the addition bathroom and need to reconnect the sewer plumbing.
I'm assuming a T connection would cause plugging issues again.