Disadvantages of using the same Steam Mobile Authenticator on two accounts?

Market limitations , ie 200 transactions(selling) unless you provide them your SIN

If you lost your phone, you lose all the accounts.

Oh, I have not tried it before, but I think you can only use one of the accounts to login the Steam mobile app at one time (to chat with friends or to buy games and stuff). But you can easily switch accounts. The Steam Guard should work just fine for multiple accounts.

Maybe you should try it and let us know :D

I found this reddit thread, for your info.
https://www.reddit.com/r/Steam/comments/3u84sr/is_it_possible_to_use_2_steam_accounts_for_mobile/

My concern is that you can't have the authenticator enabled on more than one phone for the same account.

Every other 2FA app, including Google Authenticator, Authy, and even VIP Access supports that, so you can have a backup phone in various situations.

i would be careful listening to anyone that says theres no drawbacks.

just because there isnt anything YET (because the whole mobile app thing was a big enough pill to swallow on its own), doesn't mean more big changes arent coming... expect in the future to have some form of limitations on any linked accounts that were linked after the first account.. i suspect valve will be able to identify and treat those as secondary accounts before long.

anyone who says this is bs, clearly has a trouble with pattern recognition... valve is out for blood now, and its gotten to the point where they arent even subtle about it any more.

If you get VAC ban on one account it could carry over to all others with the same authenticator / phone number.