Yes, your grid tie systems could see each other as a grid that is still running, but ONLY if they are putting enough power out to keep the voltage up. If the grid is down, and still connected, it is unlikely that you would be putting enough power out to keep the grid up, so if the grid is connected they should see it go down and act appropriately.
If the grid is disconnected, each unit should find the other is unwilling to accept any current, if they are not under load, on the other hand, one unloaded unit may prop up the loaded unit. In theory they should try to help each other, within the limits of available power. If they can't find a happy balance, one or both will shut-down using whatever fail safe they have.
It is possible that one will not like the quality of the power from the other, especially if the sine waye isn't as pure as they need for syncing up. In that case they should simply shut down.
However no matter what, you should discuss your intended design with the manufacturer, to be sure they don't have a problem with what you are trying to do. There may be some additional equipment, connections or precautions to be taken.
I have found grid tie systems to be incredibly intelligent, and reliable. I used them when I was building a critical lab that needed quality power, and I used them without solar inputs, rather I had one set being used to charge battery from the grid, and generator as backup, with a second set that simply ran off the battery. Clearly I had both grid-tie AND battery storage capabilities, I know many newer grid-tie systems don't support battery. No matter the issues around any power outage, my battery backed power source was solid with zero interruptions for 15 years straight. All maintenance, including periodic refresh of battery, installing new generators, replacing pole transformers, and even weeks long power outages were all handled by the system gracefully with no issues. I was using Trace inverters, purchased around 1996. Trace has changed names, and has newer models now, but they were so darned flexible, and so much cheaper than the more traditional systems for lab/datacenter power... They had three inputs, two AC inputs and one DC input and I could current limit each input to match the source capability. the DC and City connections were bi-directional, and the generator input was input only, at least that is how the software worked. In practice I had inter-tie functionality turned off, except when I was doing off-grid experiments that mimicked your scenario.
I did play with them when I first bought them, just to answer questions like yours, and I was very impressed with their capabilities. If I was building a power plant again today, I would very likely try to use the same type of equipment. Of course you have to be an engineer to know what you are doing, and know the electrical code well enough to stay safe.
Will your systems work as well as the systems I used? Who knows? So your mileage will vary. Not all systems have the same features that the Trace units had in 1996.