Reason is the surge of amperage when both fans hit at the same time, especially when they are worn with age and draw more current then normal. Thats when fuses blow and the fuse box eventually melts.
Wiring them to start independantly means only one at a time surges the current and all is safe - most folks would add a new fuse ( and maybe relay) for each one as well - so at least one fan works if a fuse blows.
Hope this helps,
Andy