Will I be able to understand Urdu?
Will I be able to understand basic Urdu if I learn the Urdu script after completing the Hindi course?
This is assuming you learn Hindi to a good level beyond the Duolingo course (given it's very short). I cannot speak based on how much the Hindi course teaches, as I don't know that.
You will likely have minimal difficulty understanding colloquial Urdu, as some words are different in Urdu.
The formal language however is much different from Hindi.
The answer is Yes, you will be able to understand Urdu. I am a native Urdu speaker and i speak with my Hindi speaking friends without any problem. However, i learnt Devanagari script few years ago and now when i read Hindi fiction or newspapers, i can understand 70-80% of it as in written form there are a lot of Sanskrit words so sometimes it's very different than spoken Hindi.
In short, once you know Hindi, you will be able to communicate to anyone who speaks Urdu in colloquial settings.
The languages were considered identical a mere 70 years ago before they acquired religious dimensions. Even today, most linguists argue that Hindi and Urdu are registers of the same language---Hindustani. A speaker of Bazaar Hindi could easily understand (and speak) colloquial Urdu while she may not be able to so with some so-called "dialects" of Hindi including Bhojpuri and Haryanvi.
A lot of the basic vocabulary is the same. A couple hundred years ago, it was just one language called "Hindustani" written in different scripts. Recently, there have been more deliberate efforts to create distinctions but the core is the same. If you have a firm grasp on Hindi and another Arabic script based language, you should be able to pick up Urdu easily in my opinion. As far as the basics are concerned, its the same language written in different scripts but like OmegaGmaster pointed out, the formal vocabulary is very different and takes more skill to get right.
Man I have a friend his mother tongue is Urdu and mine is Hindi and we basically soeak the same thing with a few differences but we still can understand I tgink thats because we sprka informal and slang but formal urdu is different to formal Hindi somehow I can understand formal Urdu sometimes but some people can't.