It is not possible to input something into a multiple selection with Alt+Enter.
Steps to Reproduce:
1. Make a new spreadsheet
2. select A1:A2
3. additionally select C1:C2 (hold Ctrl to do it with the mouse).
4. type something and press Alt+Enter
A dialog appears that says "Insert into multiple selection not possible". The input is only put into the current cell.
The input should be put into all selected cells.
User Profile Reset: Yes
It is possible to copy a cell and paste it into a multiple selection.
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:50.0) Gecko/20100101 Firefox/50.0
There's a possibility now to input into consecutive multi selected cells with Alt+Enter. Note: in MSO it's Ctrl+Enter.
This is enhacement request to add also possibility to input into separated multi selected cells with Alt+Enter.
I'll set this as New with low importance.