The answer to your question is really a yes and a no, it really depends on what functions you are looking for. I will start off by saying i have the Apex and have had it since it first came out, and I had the older one before that, so I am a fan of Neptune and the Apex. I also have the DOS, and the DDR.
So, yes I believe you could get two different dosing containers and install float switches or optical sensors, to alert you when the containers are low, but not through the DOS, it would be similar to a ATO set up, through a breakout box. There maybe a different way with the new modules though.
However, that is not the full function of the DDR, and if you ask me, this is where Neptune has failed, and intentionally, in order to make people purchase the DDR. The optical sensors are there to alert you when the container is running low. They are not measuring the fluid in the container they are simply acting as float switches. They will not tell you exactly how much is left in the container, or display the tile (diagrams of the containers in Fusion) that show you the containers and the fluid as it is being dosed. Unfortunately, that function is tied into the DDR and not to the DOS or Fusion. If Fusion knows how much you are dosing with the DOS, then why cant they make a function that simply subtracts the amount dosed from the starting amount.
I see no reason why they couldn't add it as stand along function of Fusion, other than to force people to buy the DDR. Even if it is a hardware problem and not a programming issue, then why not sell a cheaper module similar to the one that powers the DDR (without the containers) that can be used with any container. For example, there is nothing stopping me from leaving my DDR hooked up and using larger containers for the actual two part, and I could simply increase the starting volume amount and Fusion will subtract what is being dosed.
I would note, I haven't updated by latest firmware, so I don't know if they have finally added this function, but I doubt they did.
Another issue with the DDR that I found is with the optical sensor in the ALK container often becomes clouded as you dose. A film forms over the sensor and prevents it from tripping. I am not sure if other people have this issue, but I do constantly.
I would add, I do like the DOS itself, its very robust, I haven't had any issues with it, and its very accurate and easy to use.
One thing I did do when I got the DDR was switch to B-Ionic, its is more concentrated than regular two part, (I think it requires about 1/3 less), so it allows the containers to last longer.
One final note, is you might want to look at Building an Obsession, they make containers, I have one that I use for MG, they are larger than the DDR and they also have a float switch option built in for small up charge.
