The VAST v3.0 is corrupt at this point, as its says in ...

2.2.6 The <Creatives> Element
Each nested <Creative> element contains one of: <Linear>, <NonLinear> or <CompanionAds>. Section 1.2 describes the different Ad types.

... but should say ...

2.2.6 The <Creatives> Element
Each nested <Creative> element contains one of: <Linear>, <NonLinearAds> or <CompanionAds>. Section 1.2 describes the different Ad types.

When you read further you will find some more clarification ...

2.3.4 NonLinear Ad Format
One or more <NonLinear> ads may be included within a <NonLinearAds> element.

Conclusion: Yes, there is a way of showing multiple non-linear ads at the same time (by adding them to the VAST), but it depends on the implementation of the player, if they will be displayed. The specs telling us to implement it, but you will not know if you don't try :)

PS: Nonlinear ads usually cover the top or bottom fifth of the content video and are typically text or static images that display for about 10-20 seconds.
Top or bottom is worth considering what happens, if there are more than 2 non-linears. But as always with VAST, it's in the hand of the player architect.