I'm designing a device with a small current draw that will mostly sleep in a very low power mode and I need to use button cell batteries due to a limited thickness. I would like to double time between batteries needed to be replaced by using two or more CR2032 batteries connected in parallel, can I just connect them in parallel directly or do I need additional circuitry to prevent charging or discharging one battery from others while the device is sleeping?
If this is for mass production, then no, you cannot do this. It is never recommended to parallel primary (non-rechargeable) batteries. The reason is that if a charged one is connected to a discharged one, the current will flow into the discharged one, and charging primary cells is a big safety no-no.
If you can devise a circuit method to prevent charging of the cells under all circumstances, then you can do it. (For example, maybe you can accept the voltage drop of a Schottky diode). But otherwise, use a higher capacity button cell. There are many varieties out there.
If you are just goofing around at home, then by all means go ahead. But you should make a point of removing both batteries before you replace them. You might consider adding a resistor in series with each battery to limit the equalization current, just in case you accidentally put a fresh one in parallel with an old one.
Yes, you can definitely put them in parallel. It's a very common practice to do so. No special circuitry is necessary. If they are truly in parallel, both batteries will share the load evenly and there should be very limited (nearly zero) cross-feeding between the batteries.
Obviously, the two batteries should be at the same state of charge when you install them. Otherwise, the one with the higher voltage will initially attempt to charge up the other, which isn't desirable.
Don't fear it!
Even for Automotive products (remote keys) there are series products (remote keys) that have two CR coins batteries in parallel.
Let me explain:
In most parts of the world, these cells have to comply with certain standards, requiring some safety, e.g. they have to be short circuit safe. It's not a problem for standard CR coin cells, as they will heat up until the electrolyte starts "boiling" (around 80°C); then they'll not be able to deliver any more heating current. (Be careful with cheap "China" products - watch out for UL-certification or similar.)
So, what can happen when connecting two batteries in parallel:
The good case: connecting matching batteries (same brand and lot, both new - or similar discharge state):
1. connecting correctly with same polarity: there might be a little balancing current at the beginning only, and the discharge will always distribute evenly among the parallel batteries.
2. if you connect one battery with reverse polarity: that's like a short circuit of two batteries in series, i.e. both will be discharged fast, with self heating up to ~80°C.
The bad case: the two batteries are significantly different in charge (e.g. one fresh and one empty)
3. both same polarity: the stronger battery will charge the weaker battery in this case, which may lead to internal short-circuit (from dendrite growth) of the weaker battery, leading to heating again and depleting also the strong/fresh battery.
4. polarities different: that's the biggest problem, as in this short-circuit-discharge of the two batteries in series, the weaker battery will be empty first, while the stronger battery will still drive current, leading to an over-discharge (reverse-charging) of the weaker battery, that can make it leak due to dissolved housing. (This is what you sometimes see in devices with multiple batteries in series when you do not switch off...)
I.e. all scenarios, except #4, should be safe; for that case usually there is a mechanic reverse polarity protection (in worst case leading to a short-circuit of both batteries).
So, you have to decide how scatterbrained you might be handling your batteries.
Of course, you are free to add some protection circuitry to your device; I've seen this in the field also.
Alternatively you could use 1x CR2450 instead of 2x CR2032...