Well, if you intend to put that to the test here, I suppose I cannot stop you.

[It isn't his problem. He won't snitch about it, he'll just let it happen and wander off and say later they took separate routes out.

For the moment, though, he hefts his end of the banner and sets to work securing it; his height is, in this case, pretty beneficial. And as he works, he idly throws out:]

I expect they'll have some time to discuss our intended message, if you'd care to listen in. They ought to find it more difficult than expected to access this balcony from the inside and take it back down.

[...which may well explain the noises in there earlier, before he joined Astarion outdoors.]