Running two brands on the same format has its advantages and disadvantages. I tend to obsess about the down sides, but then I forgot about the good.
The bad;
Some lens features do not cross over.
This is sometimes a good thing. For example, the super touchy aperture ring on the 15mm Leica, something you cannot disengage on a Panasonic, is dead, i.e. not a major pain the ass on an Olympus, but also some switches and in lens I.S. are also not transferrable.
Some Flash features do not transfer.
Like the lens thing, but less of an issue off-brand units.
Things turn the wrong way (depending on how you look at it). On the Oly cameras, you can change the lens focus direction, but only in focus by wire and you are stuck with zoom rotation. My fix for this is (now) to only use Oly cameras at the paper in AF, Panasonics for my video/commercial kit and always give myself a reasonable adjustment period between the two. I also tend to use these cameras differently*, so that helps.
Panasonic AF sucks with non Panasonic lenses.
It is not unusable and for short lenses I rarely notice any issues, but for long lens sports, it is twitchy, jumpy and generally not happy. Results can be had, often more than you would assume, but without a dedicated Panasonic lens, you are in for a ride. The newest Olympus 40-150 f4 seems the best followed by the 75mm, the older 75-300 kit is by far the worst.
Using my shorter Panasonic lenses on Olympus cameras is rarely an issue. This also goes for my Sigma prime, which works seamlessly on an Olympus, but are noticeably less comfortably on a Pana.
The good.
Some of my favourite results come from mixing lenses and cameras. Generally the Panasonic cameras have lighter and brighter images, lime greens, warmer skin tones, but the Olympus cameras also vary a little, so I effectively have three looks.
This also goes for the lenses, some Olympus being warmer than others, the Panasonic range seemingly is more consistent.
Favourite combinations;
G9 and 75mm. Used for many studio portraits since I gambled on the combo for the Telstra shoot, even using the G9 for the first time for stills. I believe this combination is in my top tier for quality overall.
G9 and 45mm. The little version of above. The 45mm Olympus is optically a less delicate and lush looking lens than the 75mm, but the G9 adds that in.
OM10 or G9 and Sigma 30mm. Just very nice. I do not like the files as much from the EM1’s or the Pen F with this lens, which seems to need a little warming up, but the delicateness of the lens and G9 mesh very well.
EM10 and Leica 15. This combo just seems to rock. I used it a few times for school shoots as the third camera and regularly found the images spectacular. The EM10’s are quite warm, the lens cooler hued and the two seem to balance out, especially in mixed lighting and the AF with screen touch focus is snappy and sure footed.
EM1’s and the 9, 12-60 and 8-18 Leica’s. Delicate looking Pana lenses with extra body from the Oly sensor.
G9 and 40-150 f2.8. Not for focus tracking, which is quite frightening, but image quality.