Arduino power sharing
If you are talking about the dedicated 5V connection available on the Arduino (i.e. from the regulator, not the micro) then as long as you don't exceed the maximum current available you can power as many things as you like from it.  

To be clear, I'm assuming you are talking about the header pin the red arrow is pointing to.



How much current can this pin supply?

Well, looking at the schematics for a few Arduinos, they all seem to use the NCP1117 5V regulator in SOT-223 format. The regulator can supply up to 1A if heatsinked adequately, but since there is no heatsink present on the Arduino the capability will be much less than this.
If you use the USB supply, then it bypasses the regulator and you are then limited to the maximum USB current (500mA for USB 2.0)  

For the NCP1117, if we look at the graph for surrounding copper length, we can see that the maximum power can be between ~0.6W and 1.5W, depending on how much copper is used. At 5V, this equates to 0.6W / 5V = 120mA, and 1.5W / 5V = 300mA.   



Note this graph is for an ambient temperature of 50 degrees, so under normal conditions (~25 degrees) the maximum power will be a bit higher.  

Some of this goes to powering the ATmega, if we assume it will be less than ~100mA then that may not leave too much to power anything else if it's running at high speed, driving a few LEDs, etc. If you are not doing much, it will probably be around 10-20mA (see ATmega328 datasheet for exact figures)
Hopefully there is plenty of copper used though, so you will have a couple of hundred mA spare (more/less depending on what the ATmega is doing)
The NCP1117 has a thermal shutdown protection, so the worst that happens if you draw too much current is it will get to a certain temperature (~175 degrees - be cautious about touching it!) then shutdown until the load is removed.  

So, if your ultrasound units require less than, say 100mA combined you should probably be fine. Just give it a try and see how it goes - you can monitor the regulator temperature if you have a temperature sensor (if not you can estimate by touching briefly - if too hot to keep finger on it's >50 degrees. If you wet finger, touch and it sizzles then it's probably over 100 degrees)
Note that I have never personally used an Arduino, this is just from a quick look at the information available.