Is it possible to stack the two Pi's top of each other and connect them together somehow e.g. ethernet, to improve performance of a server for example. It's just that I have the Pi 3B sitting around collecting dust after the purchase of the latest Pi 3B+.
Sure, one can combine two RPis to share tasks between them. But simply stacking them together is not the way to go.
- it doesn't make much sense to short the gpios of the two pi's. In the worst case (both shorted gpios are configured as outputs) it will harm your RPis
- the RPi has a 3V3 voltage regulator onboard which do not provide the exact same voltage. Connecting the outputs of those regulators is bad practice
If you want to combine them you should figure out the interface that meets your requirenents (I2C, UART, SPI, ...) and simply wire the RPis in the respective way. But do not stack them.