(Welcome to the forum.) As a general rule, when the SG gets down around 1.000, the fermentation either almost finished or sometimes already finished. At the time, the yeast are not putting off as much CO2, so that when you have a large air space (ullage) you risk ill effects from oxygen because you might not have the protective CO2 layer. To reduce the risk, it is common practice to keep the fluid level close to the top of the carboy. There are several options: combine into one appropriately sized carboy; put in two different size carboys; top up with a finished product of the same type; or put sanitized marbles in one of the carboys to displace the liquid. (Oh yeah, some people buy a CO2 or argon gas bottle and put a layer of gas in the carboy.) I've done both on different batches, topped up with a finished wine and used marbles. You might see if your mead will fit in one 1-gallon and one 1/2-gallon.
