Can I safely use (2) 40 W fluorescent tubes in a 70 W fixture?
First, the internet is wrong, bulbs will fit that won't work.  Especially with T12 sockets which are the same for T12, T8 and several wirings of LED conversion.  Some LED conversion wiring is violently incompatible with real fluorescent tubes or even other LEDs.

For instance a T12 on a T8 ballast will be very dim and flicker.  

Now the way you find out what works is: 


Pop the cover off the ballast compartment.  
Look at the sticker and Read the list of tube sizes permitted.  Most likely either F40T12 or F32T8 will be at the front of the list.  
Also get the ballast model number and search the web for the ballast's data sheet. It will have some additional lamp sizes and combos.  
Buy a compatible tube.