even that situation so many counters..

First your centreback drop deeper... and you marker both like you trying too. This is very bad coz no offiside possiblity but the easiest to manage.

Second.. since you say that it harder to mark.. this guy must be fast..
if you can match his pace.. man mark him once he get near to the penalty box line.
if you can't.. stay in the box.. and get 2 -3 meter gap all the time..
most important you must always face him instead of the passer. it is more important when he start running rather than the pass. you second objective if to watch your defensive line not the passer.


once the race is on.. cover the cross and get your gk cover the near post. now common mistake here is run after the ball.. your job is run to the same line as the ball..

the ball played should be moving away from goal otherwise gk would be able to catch that. so.. you job here guess the angle where you run back and cover back the same line as the pass. so as the passer goes inner field.. so must you. the wider the pass is the faster the flanker gets to the ball.

why not running towards the ball.
if you run towards the ball.. he can shield it and block you and infront of you and comfortable run towards goal.
if you run to the cross line infront of him.. he has to stop the ball first.
How to know to run correctly.. keep the gap. if you put a 2 meter gap. keep that as you run diagonally.