I  meant after arriving in Nepal.

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Its not good idea to have visa in nepal Airport. You will be waiting in long que. I would suggest to get visa from your departure country. If you dont have time and are planning to get visa from nepal make sure you have 2 identical photos for visa. They charge almost 600 RS for 2 photo in airport

"They charge almost 600 RS for 2 photo in airport"

That's like 8 bucks, no? (Almost about the same price as what CVS, Walgreens etc charge). 

Get your visa in Nepal. An acquaintance that I recently spoke to stayed in Nepal for 4 months and didn't even get a visa. He showed his Nepali passport at arrival and US Passport at departure. No questions asked. That said, legality issues will arise. Imagine getting sent to Nakkhu jail for having stayed in Nepal illegally.   

There are seperate counters for those arriving with and without visas, and the queue for the latter is much longer.  You can clear the immigration sooner if you have a visa but that obviously does not expedite the baggage retrieval process.  You either wait longer at the immigration, or at the baggage claim.  My suggestion is that you complete the application and have the photographs ready prior to your departure.  Also complete the disembarkation card during your flight.  That way, you can immediately join the queue while others are still filling out their application forms, and your baggage will probably be on the carousel right around the time you have cleared the immigration.  The queues can be pretty long if several flights have landed around the same time, and having the application form already completed can save you a lot of time.

I strongly advise against using two passports for the same trip.  If you plan to visit India while you are in Nepal (and return to Nepal before leaving for your country or elsewhere), then it makes sense to carry a Nepali passport FOR YOUR TRIP TO INDIA otherwise do not use it.  Use only your foreign passport for travel to and from Nepal. 

its pretty convenient to get visa from Nepal. If possible have 2 passport size photos ready to save some time over there.