You really need to post your programs so we can see exactly what you are doing.
An Arduino works very much faster than Serial data is transmitted so it can appear to do two things at once.
And, for the future, give a little thought to how your writing appears to the reader. The text in your Original Post is much easier to understand with a bit of white space - like this
i want to ask whether uno can serial.print strings while receiving strings through tx rx.
i use one uno send "aaa"to the other in the loop,
when the second receiving "a",it will send "bbb"to the first one.
when i disconnect the second uno tx to the first uno rx,i find the second send "bbb"correctly through monitor,
but when i connect the second uno tx to the first rx, it will mess up in the monitor.what's wrong with it?
thanks.
...R
Serial Input Basics