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I would like to connect two external 2.5 HDDs a Pi 4. Can the Pi 4 power two of them at once?

This is the model I have in mind: WD 2TB Elements

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Not without voiding your warranty. An RPi4 can deliver a maximum of 1.2 A to all the USB ports together. This is done to protect the USB-C connector which is rated for 3A maximum.

The disks you have in mind consume up to 1 A each. Without touching to the Pi, you'll need to use a powered USB hub or a bigger-capacity single disk. If you don't care about the warranty of your Raspberry and you're powering it externally, you can bypass the current limitation by connecting the 5V GPIO pin to the Ubat pin of USB directly (check out the semi-translucent copper wire):

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P.S. A small 5V capacitor between the USB 5V and nearby GND (I used 300 uF between the same point where the wire arrives, and one of the 4 bigger solder joins) greatly improves the stability of the USB w.r.t hot-plugging new devices. Otherwise hot-plugging a second HDD may produce a voltage dip which reboots the HDD that was already connected.

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    I have connected, at the same time, a WDElements 2TB disk, and a similar Toshiba Canvio 1TB device to an RP4, and it won't boot properly. The initial current surge for spinup for these is around 900 - 1000 mA, each, even though the steady current might be lower. Either one alone is fine.
    – Michael Harvey
    Sep 27, 2019 at 23:29
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    @MichaelHarvey Spin-up current is indeed one of the highest peaks in a drive's power consumption, but it's not the only one. Running find /media/pi/drive > /dev/null on a partition with a lot of folders and files will also get you near the max current, because you'll have the head servo running all the time in addition to the spin drive.
    – Dmitry Grigoryev
    Sep 28, 2019 at 5:31
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    @Rebroad You could plug a jumper wire on the 5V pin and an unused USB cable in one of the ports, then twist the 5V line with the jumper wire. But trust me, you will want to solder it. Get the cheapest iron for $10 if you're getting it for a single use, or just borrow one.
    – Dmitry Grigoryev
    Jan 18, 2020 at 13:44
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    @MiguelStevens I did this more than a year ago and the Pi still runs fine. Obviously, it's not safe in the sense that connecting a defective USB device will let the Pi to dump excessive current into it, however, I have some over-current protection in the power supply.
    – Dmitry Grigoryev
    Dec 24, 2020 at 14:44
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    @MiguelStevens No, I don't have any reading material, and yes, a wire allows unlimited current to flow to the USB ports, which (the current) otherwise would have to go though the current limiting chip enforcing the 1.2A limit.
    – Dmitry Grigoryev
    Dec 25, 2020 at 21:52

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