With the SDK I don't think "at the same time" is accurate - as a msg can only hold a single event per cycle.
What you're looking for here would be "held down while other buttons are already pressed" would be closer to what you can do.
How to do that -
for a simple two-button combination
CodeA=0
CodeB=0
msg=port.GetMessage()
if type(msg)="roUniversalControlEvent"
code=msg.GetInt()
if code=113
CodeA=0 'Select is released
elseif code=13 'Select is Pressed
CodeA=1
if CodeB=1
'both buttons are down
end if
elseif code=106
CodeB=0 'Play/Paused is released
elseif code=6 'Play/Paused is pressed
CodeB=1
if CodeA=1
'both buttons are down
end if
end if
end if
Something like that might work - at least on an roScreen where the UniversalControlEvent is available (as TheEndless mentioned in a different thread).