UNO as HID - Sending multiply modifiers ?

I'm trying to send some keyboard shortcuts to my pc from a remote, have managed to get everything set up, and can send lowercase and uppercase letters easy enough but having some trouble getting a couple of things to work.

Firstly volume up and down don't seem to do anything using buf[2] = 0x81 and nothing is happening

Secondly anyone have any advice for sending 2 modifiers at the same time , so alt + control for example so i can get google chrome to open.

I'm hoping to use the remote to replace as much of the keyboard and mouse as possible

Any advice would be greatly appreciated