Phrases for two methods that are different but both can solve the same task?

The word "alternative" would work here. It's pretty versatile, so it should cover your context. It can be used no matter the level of formality you want and can be used to describe a wide variety of things. Here are some examples:

• We present two quicksort alternatives for cache optimizations which work well on both balanced and unbalanced data sets.
  _{Improving Memory Performance of Sorting Algorithms}

• Is ReactOS a viable Windows alternative? Taking the OS for a test drive

• The preferred alternative to this method is orElseThrow().
  _{OptionalLong (Java SE 11 & JDK 11) - Oracle Docs}

• Our selection actually contains more than 20 libraries, as some of them are alternatives to each other and solve the same problem.
  _{Top 20 Python libraries for data science in 2018}

I would say that method1 is method2's equivalent:

A person or thing that is equal to or corresponds with another in value, amount, function, meaning, etc.

https://en.oxforddictionaries.com/definition/equivalent

It seems to me that what is salient isn't a property of a method (viz that it can solve a task that another method can solve), but rather a property of a task (that there are two methods that solve it). There's the phrase "there's more than one way to skin a cat".

There's more than one way to skin a cat.

I think this is more appropriate as it more explicitly relates to processes for achieving something instead of a more abstract comparison.

Definition:

There are many methods one may employ in achieving one's ends.

Used like so:

We can go with method A if you like but there is more than one way to skin a cat.