One of the problems that i've encountered in flying cross country in loose formation is the radio.
Unless you both have two radios, then one aircraft will be doing the communication for "a flight of two? aircraft". They will have their radio tuned to the local ATC frequency and this cannot be used for communication between the aircraft.
The lead aircraft will be doing all the navigation and therefore may not always have sight of the second one. It is easy to lose sight of the second aircraft unless they are very good at holding position and height.
If the second aircraft gets the frequency wrong when changing from one controller to another, then he will have lost communication, so I have found it useful to have a "go to" frequency in the event you get separated.
The other 'issue' is where the second aircraft needs to get the attention of the first aircraft. If they both only have one radio this is difficult unless they talk on the ATC frequency (vey much frowned upon) or pull ahead to attract attention (can be dangerous).
So with two radios it is easier. With only one then you do have to agree up front a way to attract attention or to cope with being on different frequencies.
I assume that you have also done some formation work, otherwise keep at least a 25-50 mtr distance.