I'd find that weird and rude..
It was already a generous offer to 'shout' dinner for your own birthday.
In both mine and DH's families it can sometimes be a challenge to do split bills as both my dad and IL's always try to pay the entire bill, especially when it is for one of us.
In our view while generous, in this day and age , is unnecessary.
To alleviate this we often alternate who pays to save the hassle (ie they pay this time, we pay next)
At all times though we take each other's lead - if the payee is ordering entree/main/dessert then it's acceptable to replicate. If it's just a main and drink the same etc.
I have a DH who can at times be a ' big eater' but even he wouldn't eat that much food!
Never have I been out in any social setting where someone has ordered two mains in addition to entree and dessert - that's just gluttony and completely unnecessary!