Yes, you can still split the pension for the full 2018 tax year. You will prepare the tax returns together and split the pension for 2018 as you normally would as the spouse is not deceased until 2019.
There are some rules about the signing of the T1032 form that you may need to know if CRA contacts you in regards to this pension split that can be found in this link: https://bit.ly/2OTQTjE
