That's a good idea, situationally. However, it's not a lot different than the old trick of using 3 or 3.5 inch ladder pipe hose up an aerial to create an exterior standpipe. We didn't always have pre-piped waterways on the aerials.
There is a potential problem with this evolution, though. The aerial tip is resting on the structure. Aerials are designed to be operated in cantilever (i.e. with the tip unsupported). Supporting the tip puts the center of the aerial into tension. Add enough weight (twin lengths of charged LDH, maybe?) and the aerial may be damaged or even collapse.
If the hoselines are 5-inch LDH, and the aerial is a 100-footer near full extention, then we're talking about 200 feet of charged hose. With 5-inch lines holding approximately 106 gallons of water per 100 feet, we're talking around 218 gallons of water.
At 8.35 pounds per gallon, the water weight on the aerial is around 1,820 lbs.
Most aerial ladders don't have tip ratings above 1,000 lbs. Granted, the water weight is spread out over the entire length of the aerial, but I'd be concerned about doing this evolution with the second LDH line. One should be OK. The second one might, but if this is field-expedient and I haven't trained on it prior to the fire, I'm not going to do this on the fireground with more than one line.
NO KUDOS HERE. Probably lead to a disciplinary action for us. This is clearly a no/no in our case. The manufacturer of both of our apparatus(s) specify that the aerial at no time will be bridged or loaded at the tip. The unit is not designed for this and damage will occur to the mid section. Our ladders are designed for use as a cantilver, (tip always off the roof)
So in this pic (it looks like 5" to me) so if that is a 100' aerial, fully extended at a flat angle, would two 4" or 5" LDH supply lines overload the maximum working tip load? Aluminum aerials are usually 500lb tip load. Water is pretty heavy Ben calculated that already - far heavier than firefighters. Then add on the bridge effect?
This is extremely dangerous, don't operate beneath and hope the aerial is well insured.