Can I use two Arduino digital pins in parallel to keep a higher voltage?
Yes, but no.

Yes, you can use two pins to source more current, or in your case, source less current out of each. This is a common practice, but not often used on Microcontrollers. Devices like led drivers, or ULN2803 Motor Drivers, or connecting multiple transistors in parallel. Even multiple resistors in parallel. On a microcontroller, not really designed for heavy current lifting, you still have to deal with the Voltage Droop, you have to make sure that pins connected in parallel to a single source are never in different high/low states (creating a short), and you have to consider that one pin might be stronger than the other (realities of manufacturing). It would be recommended that you place both pins on the same port, so they can be changed at the same time, minimizing any chance for a short.

BUT no, it won't really work for you. You don't say which Arduino Mini, but it really doesn't matter, the different versions all have ATMega168 or ATMega328 chips and they have similar specs, as do most microcontrollers. Output pins experience Voltage Droop. As current sourced or sinked rises, the voltage decreases or increases, depending on the direction of the current and the voltage level.

The two things you need to see is the DC Characteristics for Voh (Voltage Output High), and the Pin Driver Strength.





They do not show characteristics for all VCC levels, but 2.7v and 3.0v are closer to your VCC of 3.3v than 5.0v is, so we will use those two graphs.

Notice that The test condition for VCC = 3v is that Ioh (Current Output High) is -10mA (Current sourced out, its 10mA). At 10mA sourced, the Voh is a minimum of 2.3v. That is 0.7v less than VCC.

Now look at the graph, with current on one side, and voltage on the other. When your output current at Logic High is 0mA, the pin's voltage will be at 2.7v, or VCC. At 5mA, the pin voltage will be at 2.5v. You just lost 0.2v. At 10mA, you are at ~2.2v, a loss of 0.5v.

Even if you put two pins in parallel, you are basically halving the current between the two, but assuming 8mA peak, that is still 4mA each, and that is roughly 0.2v lower than VCC. You would need a few pins in parallel, which might lead to a higher risk than you want and taking up multiple pins for no good reason.

You do not list the sensor you are using, but over all, you should either connect it directly to the 3.3v supply, or use a transistor/mosfet on a single pin if you need to have control over the sensor's power supply.