I dont think there is any way, you can give yourself all your items though (and with some more effort change it so you have all the same skills/achievements/etc. though that's a fair bit harder).
Just for the former you have to launch the game regularly through steam then go into steam/common on your pc and launch the game from its .exe file, then it's possible to host the game with the second instance and join it with the first instance tied to your steam account.
The .exe file is located by going into files, then navigate to the bottom to:
This PC -> OS -> Program files (x86) -> Steam -> steamapps -> common -> Stardew Valley then scrolling down in the stardew valley folder until you see a file with the stardew chicken icon called "Stardew Valley" which you double click to launch the game
If you have any questions please ask, it's not a super long process but it's a tad hard to explain well and I can always clarify