The heavy glider concept was the brainchild of these two men.
Udet and Messerschmitt.
Here is an early sketch made by Messerschmitt for Udet to show how a heavy glider might be towed.
The problem with multiple aircraft towing, the so called 'Troika-Schlepp' was the danger of flying into each others tows. Messerschmitt's early concept was refined somewhat, but the danger remained.
This diagram for four Bf 110s making the tow shows the problem nicely. The numbers are the length of the tows in metres.
They managed to reduce the number of tugs to three using He 111s
Or Bf 110s
To get going the Me 321 was given rockets, making a RATO.
A single Ju 90 was also used but the ultimate tug was the He 111 'Zwilling'. It was created in 1941 and entered service in 1942. It could tow a fully loaded Me 321, though rockets were needed for take off.
Odd as it looks it really was an operational aircraft by 1942. At least twelve were built.
The He 111 Z was also capable of towing two Go 242 gliders at the same time. The Go 242 was much used by the Luftwaffe.
BTW the Me 321 was the unpowered glider, the Me 323 was the powered aircraft.
Cheers
Steve