Why thoughput low when doing UL and DL at same time ?

[bluep]

Dear experts
Would you please explain more details about data thoughput in UL and DL when doing Driving test ( using data card ) in 3G and 4G, if do both UL and DL in 01 data card at the same time or 02 data card at the same time, the thoughput always low in compare with do separately UL and DL ?
Thanks a lot