I have a PreSonus AudioBox iOne interface that has two inputs and one stereo out / 2 x mono out.
Sonar only detects 1 input from this device all though it has two inputs, one for mic and one for guitar / instrument.
This makes Sonar pretty much useless for me.
Other audio programs I use detects the inputs correctly, as two separate inputs.
Please help me out with this.
 

Is it possible the input looks like this

The "Left..." input is channel 1 in your case the Mic input
The "Right..." input is channel 2 in your case the Inst input
The "Stereo..." input is both channels.
 

Nope. It just says AudioBox ASIO Driver (1 in, 1 out)
 

That is correct, there is one stereo pair in and one stereo pair out. I do not have the interface so I could only show a picture of the interface I have which has 2 stereo pairs in and out. I probably should have shown all 6 input choices in my image.
 
The "1 in" should show the submenu with a left 1 and right 1.

No, you see, that's the problem. it only picks up the 1L not 1R, so in practice I have only one input one output (mono)
 

Other audio applications registers it as two separate channels, but Sonar only gets 1... :(
 
 

So there is no submenu under "AudioBox 1in, 1out"

before you start blaming sonar for all your problems read up a little bit about your device first.
http://support.presonus.com/hc/en-us/articles/210044653-Why-won-t-my-AudioBox-record-or-play-back-audio-
 
you left little to no info about what you are using, a phantom powered mic ? a regular mic,
your signature is non existent, meaning you left no info on your PC or anything.
how are we supposed know ?
 
if you want help, leave as much info as possible.
if not then just blame sonar and use 1 input. 
 
 
EDIT: just read on presonus site ""limitation of that box is that you can only record one track at a time"
 
 

CloudsOfSound
Nope. It just says AudioBox ASIO Driver (1 in, 1 out)
 

that's because the photo he posted is for the Audiobox iTwo not the Audiobox ione like you have

the devise is also a few years old and only given Sonar 8.5 as a supported DAW in its manual.
heres the instructions from the manual.
 
Cakewalk Sonar 8.5+
1. Launch Sonar.
2. Go to Options | Audio... and click on the Advanced tab.
3. Change the Driver Mode to “ASIO.” (Using WDM, rather than ASIO, for pro-audio applications is not recommended.)
4. Click the “OK” button.
5. Restart Sonar.
6. Go to Options | Audio... and click on the Drivers tab.
7. Depending on which interface you have, highlight all input and output drivers for “PreSonus AudioBox iOne” or “PreSonus AudioBox iTwo.”
8. Go to Options | Audio... and click on the General tab.
9. Based on the interface you have, set the Playback Timing Master to “PreSonus AudioBox ... DAW Out 1.”
10. Based on the interface you have, set the Recording Timing Master to “PreSonus AudioBox ... Mic/Inst 1.”
11. You’re now ready to use your AudioBox interface.

specs
ConnectivityUSB 2.0Form FactorDesktop
Simultaneous I/O  2 x 2
A/D Resolution24-bit/96kHz
Analog Inputs1 x XLR, 1 x TS
Analog Outputs2 x TRS
Number of Preamps1
Phantom PowerYes
Bus PoweredYes
Manufacturer Part NumberAudioBox iOne
 
It would seem this device is a 2x2 interface and should support recording 2 channels simultaneously.
What do you see if you use Studio one?