Blog Post 2 Columns on Mobile

I'm trying to get a few images in a blog post to be in 2 columns on mobile.

I've successfully used this code on other blog posts like this one https://www.hvmc.sg/blog/christmas-peace (it's the profile photo at the end) to reduce the width of the top block and float it to the left so that the block below it comes up beside it (attached image).

width: 145px;
float: left;

But I can't get it to work on another post I'm doing. It's currently in draft so it's not live. But can anyone give me a generic code that I can modify for any blog post? This is what I tried but it didn't work:

#block-yui_3_17_2_1_1673946610225_4679, #block-yui_3_17_2_1_1673946610225_6394
  {
  width: 50% !important;
  float: left !important;
  }

The blocks do become 50% but remain vertically stacked i.e. they don't become 2 columns.

Hope you can understand without a live example!

Hard to tell without your website to look at. Have you tried changing the 50% to 45%, i.e. to take into account any potential padding that reduces the width to less than 100%.