Of course you can hold Lindsell Train and Fundsmith Equity Fund at the same time.
The more importnat question is: Should you, and if so, why?
First thought: what are you trying to achieve? Is it a wider spread of large, developed market shares? Why would you do this? Because you fear that Fundsmith's 25-30 shares is not a wide enough spread? Is it because you are not sure whether either fund is perfect, and want to hedge your bets in case either is not good enough after all?
So ask yourself this:
If you had £100,000 in Fundsmith, and the average holding in the fund was £3500 of your money, and one holding died completely on you tomorrow, would the total loss of that £3500 cripple your whole future?
If yes, diversify more (not just equities but bonds, cash deposits, property, gold, etc) until each unit of disaster becomes an irritating flea bite instead of a catastrophe.
Spread your asset classes so that one sector or other is likely to thrive even when others are gasping for oxygen.
Pick holdings that represent well-managed exemplars of those diversified sectors. Choose wisely, then monitor periodically that the portfolio is broadly on on target. Then go and do some gardening, paid work, or whatever helps you, yours, and others.
Only if this is not good enough should you employ fortune-tellers, and who is to say that their answers will be better than your own?