Mokuren Dojo blog, Pat Parker had an interesting article about two martial arts he practices at an advanced level Judo and Aikido. An excerpt is below. The full post may be read here.
Judo and Aikido are related arts - you might even say they are sister arts. We often go so far as to say they are the same thing (sometimes we qualify that by adding, "but not really.") This relationship between judo and aikido goes right back to the beginning - both were at least partially derived from Kito-ryu. The relationship can be seen more recently in that the judo that our group does (1950's Kodokan judo influenced by Kotani and Osawa) is highly-related to the aikido that we do (as taught by Tomiki at Waseda in the 1950's). It is difficult to examine Kodokan Goshin Jutsu and not see Tomiki/Ohba Koryu no kata and vice versa.
So, you might say that 1950's judo and Tomiki aikido are different arts with highly-overlapping domains. In a Venn diagram, they would have a large intersection set, including...
Basic posture (shizentai) is predominant instead of jigotai or hanmi
The same ukemi skills are taught and used in both
Many ukiwaza/tewaza are shared between the two arts
There are, of course elements that find themselves in the domain of one art but not the other (not intersection set). For instance, the domain of judo includes ...
greater variety of koshiwaza
...while the domain of aikido includes more...
solo exercises (tankdoku undo)
connection practice (hanasu dosa, musubi renshu)
But hang on here for a minute. There is nothing saying that these things have to be the domain of one art or the other. Why cant we start (judiciously) moving more of the material into the intersection set such that players of both arts have explicit permission to make use of it.